1. Field of the Invention
This invention relates to electrographic recording systems. Voltages are impressed on record electrodes or styluses as desired and an electrographic record medium in the proximity of the electrodes is electrostatically charged to form a latent image. Toner is subsequently applied to the record medium to develop the image. The improvement described herein relates to circuitry for impressing a voltage on the record electrodes.
2. Description of the Prior Art
An electrographic writing system may have thousands of styluses which must have a voltage impressed thereon quickly, efficiently and economically. Many different types of electrographic recording systems have been devised. For example, U.S. Pat. No. 3,662,396, incorporated by reference, discloses an electrographic recording system of the type utilizing coated paper as the record medium. A first array of recording electrodes is mounted in close proximity to the exposed surface of the charge retentive layer of the record medium. A second array of complementary electrodes is mounted adjacent to the first array in cooperative relationship with the record medium. A circuit is used to apply a first voltage of one polarity to the first array and second voltage of the opposite polarity to the second array of complementary electrodes. The second voltage thereby causes the potential of the conductive layer to change in coincidence with the first voltage.
Another type of prior art device is shown in U.S. Pat. No. 2,919,171 to Epstein and Phelps. That patent shows a record medium threaded between a recording electrode and a complementary electrode or backing bar. The record medium is moved in contact with the backing bar but not the recording electrode. A coincident voltage type of electrographic recording is utilized to charge the record medium. That is, a signal information voltage is placed on the recording electrode and a voltage of opposite polarity is placed on the backing bar. Neither voltage alone is sufficient to charge the record medium. But, when both charges are supplied simultaneously, the resultant charge is sufficient to place a latent image on the record medium.
Both of these dual electrode systems have a relatively short write time. A single electrode system is desirable to increase the time available for writing, reduce size, complexity and cost. Individual electrical drives for each stylus would impress voltages on the styluses quickly, but are economically unfeasible for the large number of styluses involved. Prior art devices have also suggested the use of a plurality of high voltage switching transistors for each stylus. The quality and expense demanded by a plurality of switching transistors for each stylus also makes this possibility undesirable.
This invention, by the use of diodes in an array, provides a switching system for an electrographic recording head which is fast, safe, compact, economical and reliable. The use of an array of diodes eliminates much of the circuitry and wiring previously thought necessary as well as a complementary charging means. Diodes and, particularly, high stored charge diodes also known as slow diodes are less costly than high voltage switching transistors and electrical drives. The use of the diode switching circuits permits relatively long writing times and the use of a wide range of record mediums. This latter feature is a distinct advantage. Previous systems often required an expensive and difficult to manufacture coated paper having a low resistance in the order of 0.25 to 1.0 megohm per square. This invention permits the utilization of a record medium having a resistance up to about 100 megohms per square. This high resistance medium is relatively easy to manufacture and inexpensive.
Increased safety to the unit itself and to any operator over previous devices is also an advantage of this invention. Prior art units used relatively high energy devices and, as a result, encountered arcing between conductors. The high voltages and currents also presented a danger to operators. This invention by the use of the diode reverse current permits the use of lower energies while still obtaining the necessary voltage to electrostatically create an image. The lower energy level significantly reduces the possibility of arcing and the danger to operators.
The diode array of this invention is usable as a switching system with the above-noted advantages because of a relatively unused phenomenon of the diode. This characteristic is the reverse current that is experienced for an appreciable time after a high stored charge diode is switched from a forward to a reverse voltage. The reverse current is used with a storage means to hold a writing stylus at a desired voltage in a system capable of charging a record medium.